The Lagarosiphon problem in Lake Wanaka : a dissertation submitted in partial fulfilment of the requirements for the Diploma in Parks & Recreation at Lincoln College

Abstract

Lagarosiphon major, an introduced oxygen weed, has been present in Lake Wanaka since 1973. An unchecked spread and increase of this aquatic weed in the lake would create further ecological change through the displacement of native aquatic plant communities. Recreational values would also be affected by dense surface-reaching weed beds growing in sheltered bays. The impending completion of the Clyde Dam and creation of Lake Dunstan on the Clutha River will provide further suitable habitat for Lagarosiphon. From 1974 a lot of effort has been spent compiling data and trying to control Lagarosiphon. It was soon realised that containment of Lagarosiphon in the southern end of the lake was a more realistic goal than eradication. However a reduction in the biomass of Lagarosiphon has given the problem a lower profile. That Lagarosiphon has been present in Lake Wanaka for 15 years without having become a serious problem, is clear evidence that current containment measures are effective. The major concerns are that Lagarosiphon does not spread any further or choke areas where it now exists. This depends upon the finance and continuation of present control measures, with surveys and reviews from time to time. (This year budget cuts have hampered the effectiveness of present control measures). The desired objective would be complete eradication of Lagarosiphon but this has not proved feasible. The plant has been eradicated from specific sites within Lake Wanaka and it is likely that sufficient resources and commitment could achieve eradication. To date the best return for effort spent has involved control of this weed through reduction of biomass.